Ion implantation is a ballistic process used to introduce into a target substrate atoms or molecules, generally referred to as dopants, to make materials with useful properties. Of particular interest, ion implantation is a common process used in making modern integrated circuits. Ion implantation may also be used for thin film deposition with controlled thickness and predefined surface properties for manufacturing optical or display devices such as flat panel displays.
FIG. 1 illustrates a conventional batch ion implantation system, of a general type which has been manufactured for about 30 years. The implantation system comprises an ion beam source 14 that forms an ion beam 13. Beam 13 is directed to impinge on a batch of target substrates 10 mounted on a disk-shaped target substrate holder 11. These elements are housed in a vacuum housing which is not shown. The disk-shaped substrate holder is spun about axis 12 (which is preferably parallel to the axis of beam 13) and translated horizontally so that the distance R between axis 12 and beam 13 is varied. In order to achieve a uniform dose of ions, the translation velocity is inversely proportional to the distance R.
In certain applications, particularly those using 300 mm wafers or larger substrates as the implantation target, it is advantageous to generate ion beams in the form of ribbon-shaped beams having high aspect ratios such that the cross-section of the beam is much larger in one dimension than the other. These ribbon beams are commonly used in ion implanter apparatus and implantation systems where a single workpiece such as a silicon wafer or flat panel display is moved in a single dimension through the ion beam. In these instances, the cross-section of the ribbon ion beam typically has one dimension that is larger than one dimension of the workpiece undergoing implantation. As a result, in one or more passes through the ion beam, a uniform dose of ions may be implanted into the workpiece.
In these applications, it is desirable that the ribbon beam have its ion trajectories moving in parallel and under careful control so as to present a uniform current density profile that is appropriate for the implantation of ions into semiconductor wafers or flat glass panels. It is also desirable that the ion beam be substantially free of undesirable species that may be present in the ion source feed material and/or in the materials of the source itself. For many years standard practice in the industry has been to use magnetic analysis to separate and reject any unwanted species or components from these ion beams. However, for large ribbon-shaped beams generally, this type of magnetic analysis and ion beam purification becomes evermore difficult and costly. This particular problem as well as the general state of the art of analyzing and transporting ribbon ion beams is reviewed in depth in White et al., “The Control of Uniformity in Parallel Ribbon Ion Beams Up to 24 Inches in Size,” Applications of Accelerators in Science and Industry 1998, AIP, p. 830, 1999, the entire text of which is expressly incorporated by reference herein.
A type of ion implantation system for silicon wafers is represented by the Models SHC 80, VIISta-80, and VIISta HC ion implantation systems manufactured by Varian Semiconductor Equipment Associates of Gloucester, Mass. This system is illustrated in FIG. 2 which is adapted from FIG. 1 of U.S. Pat. No. 5,350,926, which is incorporated herein by reference. The system comprises an ion source 2 for generating an ion beam 1, an analyzing electromagnet 3, a resolving aperture 4, and a second electromagnet 5. A control unit 36 receives beam intensity information on line 36a from a beam profiler and sends control signals along line 36b to control multipole elements in magnet 3 or along line 36c to control a beam trimmer. Magnet 3 mass analyzes the ion beam. Magnet 5 expands the beam along the dispersive plane using magnetic fields in closed loop control to maintain the uniformity of the beam. The result is a ribbon-shaped ion beam 6 that is incident on target 7. Further details of the system of FIG. 2 are set forth in the '926 patent.
Due to the complex interactions between the ion beam and the magnetic field applied for beam expansion, this approach creates severe technical, practical, and process related problems that increase the total production cost of such equipment and lead to more complicated operation procedures for carrying out the ion implantation. In particular, the beam path through this system is relatively long, and at low energies and high beam currents it becomes increasingly difficult to control the uniformity of the ion beam and the angular variation within the beam with the precision required by certain commercial processes.
It is further desirable to obtain milliampere currents of ions at energies as low as 200 eV. The highest beam currents are obtained by decelerating the ion beam immediately prior to the target. However this practice has several known disadvantages. One disadvantage is that the deceleration tends to modify the trajectories, magnifying any angular errors and making control of uniformity in a ribbon beam more difficult. Another disadvantage is that a portion of the ions is neutralized by charge-exchange processes with residual gas atoms and molecules and, as a result, is not decelerated. These ions penetrate into the silicon much further than is intended, and this deep penetration of some of the dopant ions interferes with the intended process; furthermore, since it depends on system pressure within the vacuum system, it is difficult to maintain constant conditions from day to day, and the level of contamination is not sufficiently constant to be tolerated.
In another type of application, ion beams are accelerated after traversing a mass analyzer unit. This allows a smaller and less expensive magnetic analyzer to be used, however, some of the ions are neutralized and some molecular ions are fragmented by collisions between ions or residual gas molecules. As a result, the energies of these particles are not as desired, and the ion velocities may be higher or lower than the intended velocity, resulting in implantation to an inappropriate depth. These problems are well-known, and solutions are proposed in U.S. Pat. Nos. 6,489,622 B1 and 6,710,358 B1, which are incorporated herein by reference.
In general, it is common to extend the energy range of an implanter by accelerating or decelerating the ion beam once most of the beam transport and analysis has been accomplished; however it is desirable to remove contaminant ions with the wrong energy from the ion beam, whether the energy is too high or too low.
However, these types of ion implantation systems often are not a viable solution for performing serial mode implantation with a high-current, high-uniformity ion beam that has controllable shapes and sizes. There is a need in the art of integrated circuit fabrication to provide a new system configuration, for generating a high current ion beam that has improved uniformity without requiring additional components while reducing the production cost and simplifying the manufacturing processes.